Optical information recording medium

ABSTRACT

An optical information recording medium containing a first information substrate including a first substrate having a guide groove thereon, a first recording layer including a dye, overlying the first substrate and a first reflective layer being a semi-transmittance layer, overlying the first recording layer, a second information substrate comprising, a second substrate having a guide groove thereon, a second recording layer comprising a dye, overlying the second substrate, a second reflective layer being a semi-transmittance layer, located overlying the second recording layer and a light transmittance protective layer overlying the second reflective layer and a transparent intermediate layer. Further, the following relationships (1) and (2) are satisfied: (1) 0.3≦nd/λ≦0.7, wherein n represents refractive index in the complex refractive index n-ik of the light transmittance protective layer, d represents a thickness thereof, and λ is at least one of a recording wavelength and a playback wavelength, (2) k≦0.05, wherein k represents absorption index of the light transmittance protective layer of the complex refractive index n-ik.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This document claims priority and contains subject matter related toJapanese Patent Applications Nos. 2004-081173 and 2004-343159, filed onMar. 19, 2004, and Nov. 26, 2004, respectively, incorporated herein byreference.

The present invention relates to an information recording medium inwhich information is recorded and played back with irradiation of alaser beam and more particularly to a double-layer Digital VersatileDisc (DVD) having two write-once information recording layers.

2. Discussion of the Background

In addition to read-only optical information recording media such asDVD-ROMs, recordable Digital Versatile Discs (DVD) such as DVD+RWs,DVD+Rs, DVD-Rs, DVD-RWs and DVD-RAMs have been introduced into themarket. DVD+Rs and DVD+RWs are technically positioned as an extension ofconventional recordable compact discs such as CD-Rs and CD-RWs. Tosecure compatibility between DVD+Rs and DVD+RWs and read-only DVDs, therecording density (track pitch and signal mark length) and the thicknessof the substrate of DVD+Rs and DVD+RWs are changed to DVDs from CDs. Thestructure of a DVD+R and a CD-R is similar in the following ways: a dyeis spin-coated onto a substrate to provide an optical informationrecording layer; further, a metal reflective layer is provided to thebackside of the optical information recording layer to form aninformation recording substrate; and furthermore, another similarinformation recording substrate is attached to the information recordingsubstrate with an adhesive material. In this case, a dye material isused to form the optical information recording layer. CD-Rs arecharacteristic in having a high reflectivity (i.e., 65%) complying withthe specification of CDs. To obtain such a high reflectivity for thestructure mentioned above, it is required that the optical informationrecording (absorbing) layer satisfies a predetermined complex refractiveindex for its recording wavelength and/or playback wavelength. Theoptical absorption characteristics of such a dye material is suitable tosatisfy the requirement mentioned above. This is true in the case ofDVD+Rs.

To increase the recording capacity, a read-only DVD having doubleinformation recording layers having improved optical absorption andreflection is proposed. FIG. 1 is a cross section of a DVD of prior arthaving double information recording layers. A substrate 1 and asubstrate 2 are attached with a transparent intermediate layer 5therebetween. The transparent intermediate layer 5 is formed of anultraviolet curing resin. On the inside of the substrate 1,convexoconcave layers are formed, including a semi-transmittance layer 3functioning as a first information recording layer is provided. On theinside of the substrate 2, a reflection layer 4 is formed. Thesemi-transmittance layer 3 is formed of a dielectric film or a thinmetal film. The reflective layer 4 also functions as a secondinformation recording layer and is formed of a metal layer or the like.

Recording signals recorded in each information recording layer areplayed back by using the effects of reflection and interference of aplayback laser beam. By reading signals from the two informationrecording layers, its storage capacity becomes approximately 8.5 GB atmaximum. Each of the substrate 1 and the substrate 2 has a thickness of0.6 mm. The transparent intermediate layer 5 has a thickness ofapproximately 50 μm. The semi-transmittance layer 3, i.e., the firstinformation recording layer, is formed to have a reflectivity ofapproximately 30%. The laser beam irradiated to play back informationrecorded in the reflective layer 4, also functioning as the secondinformation recording layer, is reflected at the semi-transmittancelayer 3 and attenuated in an amount of approximately 30% of the entirelaser beam. Thereafter the rest of the laser beam is reflected at thereflective layer 4 layer functioning as the second information recordinglayer and attenuated again at the semi-transmittance layer 3 and goesout of the disc. A playback laser beam is focused on the firstinformation recording layer or the second information recording layer toplayback signals therein by detecting its reflection light. Thewavelength of the laser beam for use in recording and/or playbacksignals in a DVD is approximately 650 nm.

However, the recordable DVDs, i.e., DVD+Rs, DVD-Rs, DVD-RW, DVD+RWs,etc., of prior art have only a single information recording layerreadable from one side. To increase a storage capacity thereof, it isnecessary to develop a medium which can be read from both sides. When anoptical pickup irradiates with a writing laser beam the informationrecording layer located at the back in an optical information recordingmedium in which information is recorded and read in double informationrecording layers from one side with its focus thereon there is a problemin that the other information recording layer located at the fronttherein attenuates the power of the laser beam so that the medium doesnot have a good combination of optical absorption necessary to recordinformation in the second information recording layer and opticalreflection therefrom. Published unexamined Japanese Patent ApplicationNo. (hereinafter referred to as JOP) 11-622 discloses an opticalinformation recording medium having double information recording layersformed of an organic dye in which information can be written and readfrom one side. However, this medium has just a structure of two attachedsubstrates. These attached substrates have a conventional structure of asubstrate on the incident side and a conventional structure of asubstrate on the recording layer side. Therefore, the medium does notsolve the problem of optical absorption and reflection deriving from thesecond information recording layer mentioned above. In addition, thereis no description about drawbacks in a semi-transmittance layer when thelayer is thin. JOP 10-340483 discloses an optical information recordingmedium containing a metal reflective layer, a dye-containing recordinglayer and a protective layer. The medium can use SiO and SiO₂ in theprotective layer. However, there is no specific description aboutmanufacturing conditions and optical characteristics. JOP 2000-311384discloses an optical information recording medium having a secondbarrier layer between an intermediate layer (adhesive layer) and asecond optical absorption layer. However, the literature describes onlya single example in which Au is used as a material for the secondbarrier layer. When a metal is used as in this case, the absorptionindex k in the complex refractive index increases. As a result, it isimpossible to fulfill the purpose of the present invention. Further,described in JOP 2000-311384, “absorption index k of a material for usein the second barrier layer is preferably not less than 0.1”. Thus thereis no known teaching of lowering the absorption index k to a level notgreater than 0.05.

For the foregoing reasons, a need exists for an optical informationrecording medium having double information recording layers to have alarge storage capacity in which information can be recorded and playedback from one side with good signal characteristics even for therecording layer located further from the light-incident side.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a one side recordingand playback optical information medium having double informationrecording layers in which information can be recorded and played backwith good signal characteristics even for the layer located furthestfrom the side of incident light. This object and other objects of thepresent invention will become more readily apparent and can be attainedby an optical information recording medium containing a firstinformation substrate including a first substrate having a guide groovethereon, a first recording layer containing a dye, located overlying thefirst substrate and a first reflective layer being a semi-transmittancelayer, overlying the first recording layer, a second informationsubstrate including a second substrate having a guide groove thereon, asecond recording layer containing a dye, overlying the second substrate,a second reflective layer being a semi-transmittance layer, overlyingthe second recording layer, and a light transmittance protective layerlocated overlying the second reflective layer, and a transparentintermediate layer between the first and second information substrates.The first information substrate and the second information substrate areattached to each other by an intermediary of the transparentintermediate layer with the first substrate and the second substrateoutside. Further, the following relationships (1) and (2) are satisfied:(1) 0.3≦nd/λ≦0.7, wherein n represents refractive index in a complexrefractive index n-ik of the light transmittance protective layer, drepresents a thickness of the light transmittance protective layer, andλ is at least one of a recording wavelength and a playback wavelength,and; (2) k≦0.05, wherein k represents absorption index of the lighttransmittance protective layer of the complex refractive index n-ik.

It is preferred that the optical information recording medium satisfiesthe following relationship: 0.4≦nd/λ≦0.6.

It is still further preferred that, in the optical information recordingmedium, the refractive index of the light transmittance protective layeris from 1.9 to 2.5 and the thickness thereof is from 90 to 210 nm.

It is still further preferred that, in the optical information recordingmedium, when a complex refractive index of the second recording layer isn-iK for at least one of the recording wavelength and the readingwavelength, wherein n represents a refractive index of the secondrecording layer and k is an absorption index thereof, the refractiveindex n is from 2.2 to 2.8 and the absorption index k is from 0.03 to0.07.

It is still further preferred that, in the optical information recordingmedium, the light transmittance protective layer contains ZnS as itsmain component and a transparent electroconductive oxide.

It is still further preferred that, in the optical information recordingmedium, the transparent electroconductive oxide is at least one ofIn₂O₃, ZnO and Ga₂O₃.

It is still further preferred that, in the optical information recordingmedium, the ratio of ZnS contained in the light transmittance protectivelayer based on an entire material contained therein is 50/100 to 93/100by mol.

It is still further preferred that, in the optical information recordingmedium, the light transmittance protective layer comprises multiplelayers having different refractive indices.

These and other objects, features and advantages of the presentinvention will become apparent upon consideration of the followingdescription of the preferred embodiments of the present invention takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as they become betterunderstood from the detailed description when considered in connectionwith the accompanying drawings in which like reference charactersdesignate like corresponding parts throughout and wherein:

FIG. 1 is a cross section illustrating the structure of a DVD of priorart having double information recording layers;

FIG. 2 is a diagram illustrating an example of the structure of theoptical information recording medium of the present invention;

FIG. 3 is a graph illustrating the relationship between nd/λ and thereflectivity I14H;

FIG. 4 is a graph illustrating the relationship between nd/λ and themodulation depth I14/I14H;

FIG. 5 is a graph illustrating the relationship between nd/λ and theoptimal recording power Po (mW); and

FIG. 6 is a graph illustrating the relationship between nd/λ and thejitter at the optimal recording power Po (mW).

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described below in detail with referenceto several embodiments and accompanying drawings. FIG. 2 is a diagramillustrating an embodiment of a structure of the optical informationrecording medium of the present invention. The optical informationrecording medium contains a first information substrate 20 and a secondinformation substrate 30. The first information substrate 20 contains afirst substrate 10 having a guide groove 21 thereon on which at least afirst recording layer 11 formed of a dye and a first reflective layer 12(a semi-transmittance layer) are provided in this order. The secondinformation substrate 30 contains a second substrate 17 having a guidegroove 22 thereon on which at least a second reflective layer 16, asecond recording layer 15 formed of a dye and a light transmittanceprotective layer 14 are provided in this order. The first informationsubstrate 20 and the second information substrate 30 are attached toeach other with a transparent intermediate layer 13 (adhesive layer)therebetween with the recording layers 11, 15 inside.

The first information substrate 20 has a structure containing a firstsubstrate 11, a first recording layer 12 and a first reflective layer13. This is the same as the structure of a conventional medium having asingle recording layer such as DVD+R and DVD-R except that theconventional medium has a second substrate on the first informationsubstrate. Reflectivity and recording signal modulation depth (contrast)can be obtained through a multiple interference effect at bothinterfaces of the first recording layer 11 and transformation of thefirst substrate 10 caused by mark formation. For the second informationsubstrate 30 requiring reflectivity and recording signal modulationdepth (contrast) can be obtained through the form of its guide groove 22and optical absorption characteristics of the dye. In addition, byproviding the light transmittance protective layer 14 formed of aninflexible material between the second recording layer 15 and thetransparent intermediate layer 13 formed of organic resins, etc., it ispossible to prevent the dye from flowing out due to the presence of theorganic resin and to adjust the form of marks.

In the present invention, it is preferred that the thickness of thesecond recording layer 15 is 1.5 to 2.5 times as thick as that of thefirst recording layer 11. When the difference in both thicknesses is outof this preferred range, it is difficult to record information in bothlayers 11, 15 with the same strategy (light emission pulse patterns of arecording laser beam) because recording marks spread differently on bothlayers 11, 15. With regard to dye films, typically an applicationsolvent in which a dye is dissolved is spin-coated. When such a dyesolvent is applied to a substrate having a guide groove, the thicknessof the dye applied to a groove portion and a portion between grooves isdifferent. In addition, the groove forms formed on the first substrateand the second substrate are different. It is preferred that a 4.7 GBDVD+R or DVD-R having a 0.74 μm pitch has a groove depth of from 100 to200 nm and a groove width (at its bottom) of from 0.2 to 0.3 μm. When aspin coating method is used, a dye tends to be filled in the groove.Forms formed at the interface between the dye recording layer and thereflective layer are determined by the amount of the dye filled and thegroove form of the substrate. Therefore, when reflection at theinterface therebetween is utilized, the range mentioned above issuitable.

By contrast, the groove form 22 of the second substrate 17 preferablyhas a depth of from 20 to 60 nm and a groove width of from 0.2 to 0.4μm. As illustrated in FIG. 2, since the interface form formed betweenthe dye recording layer 15 and the reflective layer 16 is determined bythe groove form of the substrate, the range mentioned above is suitableto utilize the reflection at the interface. When the groove depth of thefirst substrate 10 and the second substrate 17 is relatively deep incomparison to the range mentioned above, the reflectivity easilydecreases. In addition, when the groove depth is relatively shallow incomparison to the range mentioned above or the groove width is outthereof, tracking is unstable during recording and recording marksformed tend to vary so that jitter tends to increase.

The first recording layer 11 at the groove portions and the secondrecording layer 15 at the portions between grooves preferably have athickness of from 40 to 100 nm and from 60 to 200 μm, respectively. Whenthe thickness of the recording layer is relatively thin in comparisonwith the range mentioned above, signal modulation depth (contrast) isdifficult to obtain. When the thickness of the recording layer isrelatively thick in comparison with the range mentioned above, the marksformed tend to have different forms so that jitters easily increase.

Next, the materials for use in each layer of the optical informationrecording medium of the present invention will be described.

The optical information recording medium of the present invention has astructure in which a high reflectivity can be obtained by multipleinterference effect at both interfaces of a recording layer containing adye as in the case of a DVD+R and a CD-R. The recording layer containinga dye is necessary to have optical characteristics such that therefractive index n and the absorption index k in the complex refractiveindex n-ik are large and relatively small, respectively, for a recordingwavelength and/or playback wavelength λ (nm). The refractive index n isgreater than 2 and preferably from 2.2 to 2.8. The absorption index k isfrom 0.02 to 0.2 and preferably from 0.03 to 0.07. When the absorptionindex k is too small, the sensitivity of the recording layer tends todeteriorate because the absorption of a recording laser beam thereat isnot good. When the absorption index k is too large, the reflectivity atthe recording layer tends to deteriorate so that it is difficult toobtain sufficient reflectivity at the recording layer located furthestfrom the light incident side when the medium is a double recording layertype. Such suitable optical characteristics are obtained by utilizingcharacteristics at the end portion on the long wavelength side inoptical absorption band of the dye layer. The optical informationrecording medium of the present invention is for a red laser beam havinga wavelength of from 600 to 800 nm and preferred writing and/or readingwavelength λ of from 650 to 670 nm. When designing a medium, thewavelength of a writing and/or reading laser beam is determined to be inthe range mentioned above and then materials for and the thickness ofeach layer are selected under the conditions of the present invention.

Specific examples of dye materials for use in the first and secondrecording layers include cyanine dyes, phthalocyanine dyes, pyrylium/thiopyrylium dyes, azulenium dyes, squarilium dyes, azo dyes, formazanechleate dyes, metal complex dyes (e.g., Ni and Cr),naphthoquinone/anthraquinone dyes, indophenol dyes, indoaniline dyes,triphenyl methane dyes, triallyl methane dyes, aminium/diinmonium dyesand nitroso dyes. Among these, considering the dye layer formingproperty and adjusting property of optical characteristics,tetraazaporphyrin dyes, cyanine dyes, azo dyes and squarilium dyes arepreferred as dyes which have a maximum absorption wavelength of opticalabsorption spectrum of from 580 to 620 nm and which can easily obtainpredetermined optical characteristics for a laser beam wavelength forDVD, i.e., approximately 650 nm. In addition, such a recording layer canbe formed of a dye alone or a combination of a dye and other componentssuch as a binder and a stabilizer.

Any substrate for use in conventional information recording media can beused as the substrate of the present invention. Specific examples ofmaterials for use in the substrate of the present invention includeacrylic resins such as polymethyl methacrylate, vinyl chloride resinssuch as polyvinyl chloride resins and vinyl chloride copolymers, epoxyresins, polycarbonate resins, amorphous polyolefin resins, polyesterresins and glass and ceramics such as soda lime glass. Among these, interms of dimension stability, transparency and planarity, polymethylmethacrylate resins, polycarbonate resins, epoxy resins, amorphouspolyolefin resins, polyester resins and glass are preferred. Further,polycarbonate resins are most preferred in terms of processability.

Materials having a high reflectivity for a laser beam wavelength arepreferred for the first reflective layers (semi-transmittance layer).Preferred specific examples of materials therefor include metals andhalf metals such as Au, Ag, Cu, Al, Ti, V, Cr, Ni, Nd, Mg, PD, Zr, Pt,Ta, W, Si and Zn. Among these, it is preferred to use an alloy mainlyformed of one of Au, Ag, Cu and Al to which at least one of Au, Ag Cu,Al, Ti, V, Cr, Ni, Nd, Mg, PD, Zr, Pt, Ta, W, Si and Zn other than themetal selected as the main component is added in an amount of 1 to 10%by weight. By adding such a metal in an amount of at least 1% by weightto form such an alloy, crystalline particles thereof are finely minuteso that a thin film having excellent corrosion resistance can beobtained. Too large an additional amount is not preferred because thereflectivity at the reflective layer decreases.

The first reflective layer preferably has a thickness of from 5 to 30nm. When a recording layer containing a dye and a transparentintermediate layer formed of an acrylic resin, etc., is in contact withan extremely thin first reflective layer having a thickness of 30 nm, itis necessary to prevent the dye and the acrylic resin from movingthrough the reflective layer and reacting with each other. When areflective layer is formed of an unalloyed metal thin layer containingcrystal having a large particle diameter, such a thin layer tends to beisland-shaped and the acrylic resin easily infiltrates from the grainboundary.

A light transmittance protective layer 14 is provided between the secondrecording layer 15 and the transparent intermediate layer 13 tochemically and physically protect the recording layer 15 containing adye. Specific examples of materials for use in such a lighttransmittance protective layer 14 include oxides such as SiO_(x) (x is 1or 2), In₂O₃, SnO₂, ZNO, Ga₂O₃, Al₂O₃, MgO, TiO₂, Ta₂O₅, half metals orsemiconductors such as Si, Ge, Sic, Tic and graphite, fluorides such asMgF₂, AlF₃, LaF₃ and CeF₃, sulfides such as ZnS, CdS and Sb₂S₃, nitridessuch as Si₃N₄ and AlN, ZnSe, GaSe, ZnTe and their combinations. Amongthese, materials containing a large amount of compounds having a smallinternal stress such as ZnS, CdS, ZnSe, ZnTe, Sb₂S₃ and SiO_(x) arepreferred. Further, to optimize the refractive index n and theabsorption index k, mixtures of these compounds can be used. Thesematerials have a high melting point. When materials do not read witheach other during sintering of a target, the refractive index n and theabsorption index k are almost equal to the weighted mean of theirmixture ratio.

Especially, ZnS is suitable to improve productivity and to reduce costbecause ZnS is less toxic and less expensive and has a high sputteringrate. The mixture ratio of ZnS is preferably from 50 to 93 mol % andmore preferably from 70 to 91 mol %. When the mixture ratio of ZnS istoo high, a thin layer is not formed well on the second recording layercontaining a dye. To adjust the refractive ratio n, the mixture ratio ofZnS is determined to be not greater than 93 mol % and preferably notgreater than 91 mol %. In addition, it is good to mix ZnS with amaterial having a different refractive index. Too low a mixture ratio ofZnS is not preferred because one of the good characteristics of ZnS,i.e., high sputtering rate, is degraded. When a thin layer of a mixtureis formed, it is possible to sputter plural targets simultaneously.However, this is not preferred because equipment cost rises andcontrolling the mixing ratio is difficult. Therefore, it is advantageousin terms of productivity to manufacture a mixture of ZnS and additivesbefore sputtering. The refractive index of ZnS is approximately 2.35. Todecrease the refractive index, ZnS is mixed with SiO₂. Such a mixturecan be used for a target for CD-RWs, DVD-RWs and DVD+RWs currently inthe market so that the quality of obtained media is stable.

In addition, it is possible to increase the refractive index of ZnS byadding, for example, Si (n-ik=4.2-i0.3), SiC (n-ik=7-i0.1), SiO(n-ik=1.95-i0), SiO₂ (n-ik=1.46-i0), SiO_(x) (x is 1 or 2) (n-ik=1.46 to1.95-i0), TiO₂ (n-ik=2.6-i0) or Ge (n-ik=5.3-i0.5) in an amount not lessthan 5 mol %. When the addition ratio is too small, the effect ofincreasing the refractive index n is ignorably small.

It is possible to add Si in an amount of 30 mol % at maximum so that therefractive index n can be raised up to approximately 3.2. When Si isadded too much, the absorption index k becomes too large. This is notpreferred because the reflectivity decreases. It is possible to raisethe refractive index n to approximately 2.5 by adding SiC in an amountof 40 mol % at maximum. When too much SiC is added, the stress increasesso that the recording layer and the light transmittance protective layerdetach at their interface. In addition, electroconductivity can be addedto a target by adding a transparent electroconductive oxide such asIn₂O₃, SnO₂, ZnO, Ga₂O₃ and Nb₂O₃ or an oxide in which an element suchas Al, Ga, In Sn, Nb, Zn, Ta, F and Sb is doped to the transparentelectroconductive oxide. Thereby, DC sputtering is possible. That is,the sputtering rate increases. This contributes to shortening of theproduction takt time and cost reduction of production equipment. To useDC sputtering, a target is necessary to have a specific resistance notgreater than 1 Ωcm, preferably not greater than 0.1 Ωcm to prevent aproblem such as arc even when a sputtering power is high, and morepreferably not greater than 0.01 Ωcm to unnecessitate an arc cut deviceor pulse overlapping device to a sputtering power source, resulting incost reduction of production equipment. When the SiC addition amount istoo much, the stress of a film layer increases and thus the recordinglayer and the light transmittance protective layer detach at theirinterface. Therefore, the SiC addition amount is limited to 50 mol % atmaximum.

Further, it is possible to provide double light transmittance protectivelayers to secure cohesiveness of a recording layer and a lighttransmittance protective layer. A first transparent protective layer,which is soft and contains ZnS in a relatively large amount, is providedon a dye recording layer and a second transparent protective layer,which is hard and contains ZnS in a relatively small amount, is providedon the first transparent protective layer to prevent deformation of thedye recording layer caused by recording. Thereby, crosstalk betweenadjacent tracks can be restrained.

The layer thickness of a light transmittance protective layer ispreferably from 10 to 300 nm, more preferably from 90 to 210 nm andfurther more preferably from 95 to 200 nm. When the layer thickness istoo thin, materials in the transparent intermediate layer infiltratesinto the second recording layer through defective portions in the lighttransmittance protective layer, resulting in deterioration of the dye inthe recording layer. When the layer thickness is too thick, thetemperature of a substrate during sputtering greatly rises, resulting inan increase in the layer stress. Therefore, the substrate tends todeform and the light transmittance protective layer easily detaches. Inaddition, when the absorption index k is not zero, the reflectivity atthe reflective layer decreases by absorption.

As illustrated in FIGS. 3 to 6 described later, by determining nd/λ from0.3 to 0.7, jitter can be limited to not greater than 11%, which is nota problematic level when information is recorded or played back by a DVDplayer. In addition, by determining nd/λ from 0.4 to 0.6, a medium canhave a reflectivity not less than 15%, a modulation depth not less than0.6 and a jitter not greater than 10% so that playback compatibilitythereof can be improved. Further, the absorption index k can bedetermined to be not greater than 0.05 so that it is possible to have areflectivity not less than 11%, which does not cause a practicalproblem.

Therefore, the absorption index k is determined to be not greater than0.05 and nd/λ is determined to be from 0.3 to 0.7, preferably from 0.4to 0.6 and, more preferably from 0.4 to 0.5, wherein λ representsrecording wavelength and/or playback wavelength for a lighttransmittance protective layer and wherein n and k in the complexrefractive index n-ik represent refractive index and absorption index,respectively, and d represents layer thickness of thelight-transmittance protective layer.

The relationships among the complex refractive index, the layerthickness and the reflectivity of a second recording layer are asfollows.

(1) The reflectivity does not fluctuate when the refractive index n ofthe second recording layer is changed but the frequency of thereflectivity change is short when the thickness of the lighttransmittance protective layer is changed;

As the layer thickness of the second recording layer increases, thereflectivity decreases. The frequency of the reflectivity change isshort when the thickness of the light transmittance protective layer ischanged; and

The reflectivity decreases when the absorption index k of the secondrecording layer is changed but the frequency of the reflectivity changedoes not change when the thickness of the light transmittance protectivelayer is changed.

The same material as mentioned for a first reflective layer can be usedfor a second reflective layer. The thickness of a second reflectivelayer is approximately 120 to 160 nm.

It is preferred that a transparent intermediate layer also functions asan adhesive layer. Specific examples of materials for such a transparentintermediate layer include conventional acrylate, epoxy and urethaneultraviolet curing resin or thermosetting adhesive agents. In addition,a transparent sheet can be used as an adhesive layer. The thickness of atransparent intermediate layer is from 40 to 70 μm and preferably from50 to 60 μm.

In addition, acrylic or epoxy ultraviolet curing resins, etc., can beinserted between a light transmittance protective layer and atransparent intermediate layer to protect the layers chemically orphysically.

Having generally described this invention, further understanding can beobtained by reference to certain specific examples which are providedherein for the purpose of illustration only and are not intended to belimiting. In the descriptions in the following examples, the numbersrepresent weight ratios in parts, unless otherwise specified.

EXAMPLES Examples 1 to 4

A coating liquid in which a squarilium dye compound represented by thefollowing Chemical formula 1 was dissolved in2,2,3,3-tetrafluoropropanol was coated on a polycarbonate substrate(first substrate) having the following dimension to form a firstrecording layer.

Dimension of the polycarbonate substrate:

diameter  120 mm thickness 0.58 mm

Guide groove having convexoconcave patterns on the polycarbonatesubstrate

depth  140 nm width 0.25 μm track pitch 0.74 μm

The maximum absorption wavelength of this recording layer was 609 nm andthe absorption level was 0.6 at the maximum absorption wavelength. Onthe first recording layer, a first reflective layer having a thicknessof 9 nm formed of Ag_(99.5)In_(0.5) was formed under the followingconditions:

Sputtering device: BIGSPRINTER (manufactured by Unaxis Co. Ltd.)

Sputtering gas: Ar

Sputtering pressure: 6.0×10⁻³ torr

Sputtering power: 3.5 kW

A second reflective layer having a thickness of approximately 120 nmformed of Ag₉₈Cu₂ was formed on a second substrate having the followingdimensions using BIGSPRINTER (manufactured by Unaxis Co. Ltd.) as asputtering device and Ar as a sputtering gas:

Dimension of the second polycarbonate substrate:

diameter  120 mm thickness 0.60 mm

Guide groove having convexoconcave patterns on the second polycarbonatesubstrate

depth   40 nm width 0.25 μm track pitch 0.74 μm

Next, a squarilium dye compound represented by the following chemicalformula 1 was spin-coated on the second reflective layer to form asecond recording layer having a thickness such that absorption level ofthe second recording layer is 1.1. Further, GeS₂ was sputtered using Aras a sputtering gas to form a light transmittance protective layerhaving a thickness of 110 nm (Example 1), 130 nm (Example 2), 140 nm(Example 3) and 150 nm (Example 4).

The first substrate and the second substrate on which the recording andreflective layers were formed were attached to each other with anultraviolet curing adhesive agent (KARAYAD DVD576M manufactured byNippon Kayaku Co., Ltd.) functioning as a transparent intermediate layerto obtain an optical information recording medium having a structureillustrated in FIG. 2.

Examples 5 to 19

Optical information recording media for Examples 5 to 19 were obtainedin the same manner as described in Example 1 except that materials andthe thickness of the light transmittance protective layer were changedas shown in Table 1.

The optical information recording media of Examples 1 to 19 wereevaluated using a DVD evaluation device DDU1000 manufactured by PulstecIndustrial Co., Ltd.) with a wavelength of 657 nm and numerical apertureNA of 0.65 in the following way: Record DVD (8 to 16) signals with anoptimal recording power Po (mW) (i.e., the recording power at which datato data dock jitter was minimum) at a linear velocity of 9.2 m/s; andevaluate playback performance at a linear velocity of 3.8 m/s. Theresults are shown in FIGS. 3 to 6. The recording pulse strategy was amultiple pulse system of (n-2)T with a multiple pulse width of 10/16.

As seen in FIG. 3, when a refractive index was large, the reflectivityafter recording I14H (%) varies in a cyclic manner against nd/λ and washigh when nd/λ was not greater than 0.25 and was from 0.4 to 0.7. Asseen in FIG. 4, 14T modulation depth, i.e., ratio of reflectivity I14 toreflectivity after recording I14H, was high when nd/λ was not greaterthan 0.07 and was from 0.3 to 0.6. In addition, as seen in FIG. 6, thejitter at the optimal recording power was not greater than 10% when nd/λwas from 0.35 to 0.6. When nd/λ was from 0.3 to 0.7, the jitter was notgreater than 11% so that there was no unacceptable error. When nd/λ wasfrom 0.4 to 0.6, the jitter was not greater than 10%, meaning thatproblems such as decrease in playback velocity did not occur even when aDVD-ROM drive or DVD-Video in the market was used for playback. Inaddition, it was possible to adjust the reflectivity after recording ata second recording layer by changing the thickness of a first reflectivelayer without affecting modulation depth and jitter.

Examples 20 to 26

Optical information recording media for Examples 20 to 26 were obtainedin the same manner as described in Example 1 except that materials andthe thickness of the light transmittance protective layer were changedas shown in Table 1. The recording characteristics were measured in thesame manner as in Example 1. The results are shown in Table 1.

As seen in Table 1, reflectivity I14 (%) at the optimal recording power,modulation depth I14/I14H (%) and jitter (%) were excellent in Examples20 to 26.

TABLE 1 Modulation Sputter depth Light trasnmittance protective powerRefractive Absorption Layer Reflectivity (I14/I14H) Jitter Po layer(composition ratio in mol %) source index index thickness nd/λ I14H(%)(%) (%) (Mw) Ex. 1 GeS₂ RF 2.13 0.001 110 0.35 15 0.81 9.0 15 Ex. 2 GeS2RF 2.13 0.001 130 0.41 17 0.89 8.2 16 Ex. 3 GeS₂ RF 2.13 0.001 140 0.4517 0.90 8.7 17 Ex. 4 GeS₂ RF 2.13 0.001 150 0.48 18 0.91 8.2 17 Ex. 5ZnS(80)SiC(20) RF 2.38 0.01 100 0.36 13 0.86 9.2 13 Ex. 6 ZnS(80)SiC(20)RF 2.38 0.01 120 0.44 16 0.93 8.3 15 Ex. 7 ZnS(80)SiC(20) RF 2.38 0.01140 0.51 20 0.87 7.7 18 Ex. 8 ZnS(80)SiC(20) RF 2.38 0.01 160 0.58 210.67 8.6 19 Ex. 9 ZnS(80)SiC(20) RF 2.38 0.01 180 0.66 19 0.53 10.4 18Ex. 10 ZnS(80)ZnO(20) DC 2.21 0.01 90 0.30 13 0.62 10.9 12 Ex. 11ZnS(80)ZnO(20) DC 2.21 0.01 120 0.40 15 0.84 8.8 13 Ex. 12ZnS(80)ZnO(20) DC 2.21 0.01 140 0.47 17 0.88 8.3 15 Ex. 13ZnS(80)ZnO(20) DC 2.21 0.01 160 0.53 21 0.79 8.6 17 Ex. 14ZnS(80)ZnO(20) DC 2.21 0.01 180 0.60 21 0.61 9.9 18 Ex. 15ZnS(80)ZnO(20) DC 2.21 0.01 206 0.69 19 0.52 10.8 17 Ex. 16ZnS(75)SiC(15)ZnO(10) DC 2.25 0.04 120 0.40 15 0.84 9.3 12 Ex. 17ZnS(75)SiC(15)ZnO(10) DC 2.25 0.04 140 0.47 18 0.88 8.6 15 Ex. 18ZnS(75)SiC(15)ZnO(10) DC 2.25 0.04 160 0.53 20 0.78 9.0 16 Ex. 19ZnS(75)SiC(15)ZnO(10) DC 2.25 0.04 180 0.60 21 0.67 9.3 19 Ex. 20ZnS(60)SiO₂(40) RF 1.9 0.005 200 0.58 21 0.67 9.6 19 Ex. 21ZnS(91)ZnO(7)In₂O₃(2) DC 2.25 0.001 140 0.48 18 0.84 8.3 18 Ex. 22ZnS(93)ZnO(6)Ga₂O₃(1) DC 2.25 0.001 140 0.48 18 0.83 8.2 17 Ex. 23ZnS(60)ZnO(40) DC 2.1 0.005 120 0.38 14 0.78 8.5 14 Ex. 24ZnS(50)ZnO(50) DC 2.05 0.01 120 0.37 13 0.75 8.8 14 Ex. 25ZnS(70)TiC(10)Nb₂O₅(20) DC 2.5 0.05 95 0.36 11 0.62 10.0 14 Ex. 26SnO₂(70)Ta₂O₅(30) DC 2.0 0.02 160 0.49 19 0.78 8.3 18

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit and scope of theinvention as set forth therein.

1. An optical information recording medium comprising: a firstinformation substrate comprising: a first substrate having a guidegroove thereon, a first recording layer comprising a dye, overlying thefirst substrate; and a first reflective layer being a semi-transmittancelayer, located overlying the first recording layer; a second informationsubstrate comprising: a second substrate having a guide groove thereon,a second reflective layer, located overlying the second substrate; asecond recording layer comprising a dye, located overlying the secondreflective layer; and a light transmittance protective layer locatedoverlying the second recording layer; and a transparent intermediatelayer, wherein the first information substrate and the secondinformation substrate are attached to each other by an intermediary ofthe transparent intermediate layer with the first substrate and thesecond substrate outside and wherein the following relationships (1) and(2) are satisfied: 0.3≦nd/λ≦0.7, wherein n represents a refractive indexof the light transmittance protective layer, d represents a thickness ofthe light transmittance protective layer, and λ is at least one of arecording wavelength and a playback wavelength, (2) k≦0.05, wherein krepresents an absorption index of the light transmittance protectivelayer of the complex refractive index n-ik.
 2. The optical informationrecording medium according to claim 1, wherein, the followingrelationship is satisfied: 04≦nd/λ≦0.6.
 3. The optical informationrecording medium according to claim 2, wherein the refractive index ofthe light transmittance protective layer is from 1.9 to 2.5 and thethickness thereof is from 90 to 210 nm.
 4. The optical informationrecording medium according to claim 1, wherein, when a complexrefractive index of the second recording layer is n-iK for at least oneof the recording wavelength and the reading wavelength, wherein nrepresents a refractive index of the second recording layer and k is anabsorption index thereof, the refractive index n is from 2.2 to 2.8 andthe absorption index k is from 0.03 to 0.07.
 5. The optical informationrecording medium according to claim 1, wherein the light transmittanceprotective layer contains ZnS as its main component and a transparentelectroconductive oxide.
 6. The optical information recording mediumaccording to claim 5, wherein a ratio of ZnS contained in the lighttransmittance protective layer based on an entire material containedtherein is 50/100 to 93/100 by mol.
 7. The optical information recordingmedium according to claim 5, wherein the transparent electroconductiveoxide is at least one of In₂O₃, ZnO and Ga₂O₃.
 8. The opticalinformation recording medium according to claim 1, wherein the lighttransmittance protective layer comprises multiple layers havingdifferent refractive indices.